Active flow control involves modification of the turbulent structure of eddies in most complex flows with the intent to improve aerodynamic performance of air vehicle flight control and propulsion systems. Such capability can increase range and maneuverability, reduce acoustic loads, signature, weight, and cost. In some systems, a relatively small amount of high-momentum secondary fluid is used to enhance the naturally occurring instabilities of the main flow. For example, it is known to use active flow control in applications such as favorably influencing the flow over an aerodynamic surface, heating/cooling components, vectoring a primary fluid flow, and mixing fluids.
One type of device that can be used for active flow control in subsonic systems is referred to as a zero-net-mass jet. A typical zero-net-mass jet actuator comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected in an external environment out from the orifice of the housing. Various volume changing mechanisms are known, for example a piston positioned in the jet housing to move so that fluid is moved in and out of the orifice during reciprocation of the piston, or a flexible diaphragm as a wall of the housing. The flexible diaphragm is typically actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is utilized to create time-harmonic motion of the diaphragm. As the diaphragm moves into the chamber, decreasing the chamber volume, fluid is intermittently ejected from the chamber through the orifice. As a quantity of fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates a shear layer, which rolls up into a vortex sheet or ring. As each intermittent quantity of fluid is emitted, a separate vortical structure is generated creating a train of vortices moving away from the orifice. These vortices move away from the edges of the orifice under their own self-induced velocity. As the diaphragm moves outward with respect to the chamber, increasing the chamber volume, ambient fluid is drawn from all directions around the orifice into the chamber. Since the vortices are already removed from the edges of the orifice, they are not affected by the ambient fluid being entrained into the chamber. As the vortices travel away from the orifice, they synthesize a jet of fluid, a “zero-net-mass jet,” through entrainment of the ambient fluid.
However, piezoelectric diaphragms used to form zero-net-mass jets are generally unreliable due to moving parts and cause vibration in devices in which they are installed. Further, the amplitude, temperature, and frequency at which the diaphragms can operate is limited, with the result that piezoelectrically-driven zero-net-mass jets generate limited jet velocity and have little practical application in flows above approximately Mach 0.3.
In physics and chemistry, plasma (also called an ionized gas) is an energetic state of matter in which some or all of the electrons in the outer atomic orbital rings have become separated from the atom. Excitation of a plasma requires partial ionization of neutral atoms and/or molecules of a medium. There are several ways to cause ionization including collisions of energetic particles, strong electric fields, and ionizing radiation. The energy for ionization may come from the heat of chemical or nuclear reactions of the medium, as in flames, for instance. Alternatively, already released charged particles may be accelerated by electric fields, generated electromagnetically or by radiation fields.
There are two broad categories of plasma, hot plasmas and cold plasmas. In a hot plasma, full ionization takes place, and the ions and the electrons are in thermal equilibrium. A cold plasma (also known as a weakly ionized plasma) is one where only a small fraction of the atoms in a gas are ionized, and the electrons reach a very high temperature, whereas the ions remain at the ambient temperature. These plasmas can be created by using a high electric field, or through electron bombardment from an electron gun, and other means . . .